1. Field of the Invention
The present invention relates to a power factor control circuit and power factor control method, and more particularly to a control circuit and a control method outputting pulse width modulation (PWM) signals in accordance with a present current mode at which a switching power supply circuit is operated.
2. Description of the Related Art
With reference to FIG. 5, to convert an AC mains (Vac) into a DC power supply and supply the DC power to a load 60, a rectifier 50 and a switching power supply circuit 70 are connected between the AC mains (Vac) and the load 60. The rectifier 50 converts the AC power into the DC power. An input terminal of the switching power supply circuit 70 is connected to an output terminal of the rectifier 50 so that the switching power supply circuit 70 converts the DC power into a more stable DC power and supplies the more stable DC power to the load 60 through an output terminal thereof. The switching power supply circuit 70 has a power factor correction (PFC) circuit 71 and a DC to DC converter 72. DC voltages having different voltage levels (for example 28V, 12V), which are converted by the DC to DC converter 72 from a voltage (for example 380V) that is outputted from the PFC circuit 71, is further outputted to the load 60.
The PFC circuit 71 has an inductor L, an output capacitor C and an active switch 711, a first proportional integral amplifier 81, a second proportional integral amplifier 82, a multiplexer 83 and a driver 84. The active switch 711 is connected between the inductor L and the output capacitor C and has a control terminal.
The first proportional integral amplifier 81 has two input terminals and an output terminal. The input terminals are respectively connected to an output terminal of the PFC circuit 71 and a reference voltage Vref1, and the output terminal is connected to one input terminal of the multiplexer 83. Another input terminal of the multiplexer 83 is connected to the output terminal of the rectifier 50.
The second proportional integral amplifier 82 has two input terminals respectively connected to the output terminal of the rectifier 50 and an output terminal of the multiplexer 83.
The driver 84 has two input terminals respectively connected to an output terminal of the second proportional integral amplifier 82 and a reference voltage Vref2. An output terminal of the driver 84 is connected to the control terminal of the active switch 711.
A conventional power factor control method in collaboration with the foregoing circuits is described as follows. The first proportional integral amplifier 81 outputs a voltage error signal Verror based on a difference between an actual output voltage of the PFC circuit 71 and the reference voltage Vref1. The second proportional integral amplifier 82 outputs a voltage signal Ierror indicative of a current error to the driver 84 in accordance with an inductor current IL and the voltage error signal Verror. The driver 84 outputs a PWM signal to the control terminal of the active switch 711 to control a duty cycle D when the active switch 711 turns on in accordance with the voltage signal Ierror and the reference voltage Vref2.
To let the PFC circuit 71 output a stable voltage Vout, the relationship among the duty cycle D, an actual voltage Vin, sense and a preset output voltage Vout can be expressed by an equation as follows.
  D  =      1    -                  V                              i            ⁢                                                  ⁢            n                    ,          semse                            V        out            
As the duty cycle of the PWM signal is obtained according to the voltage signal Ierror and the voltage signal Ierror contains the components of the actual input voltage, the inductor current and the actual output voltage, the power factor can be enhanced by adequately configuring the PFC circuit.
Whenever the actual output voltage differs from the reference voltage Vref1 and the inductor current IL differs from the voltage signal Ierror in the PFC circuit 71, the driver 84 can output a corresponding PWM signal.
A switching power supply circuit can be operated at different current modes, such as a continuous current mode (CCM) and a discrete current mode (DCM), by varying a load condition. When the switching power supply circuit is operated at the CCM, the active switch 711 turns on during the duty cycle of the PWM signal so that the inductor L is charged and the current passing through the inductor increases. The active switch 711 turns off when operating outside the duty cycle of the PWM signal so that the inductor L discharges. Before the inductor L discharges all its stored energy, the PWM signal triggers the active switch 711 to turn on again so that the inductor L can be charged to an even higher current level. During the course of repeatedly being charged and discharging, the waveform of an average current of the inductor L follows the waveform of the input voltage.
When operated at the DCM, the current of the PFC circuit 71 is relatively lower than that at the CCM. When operating within the duty cycle of the PWM signal, the active switch 711 turns on so that the inductor L is charged and the current passing through the inductor increases. When operated outside the duty cycle of the PWM signal, the inductor L discharges all its stored energy and is charged at next duty cycle so that the average current passing through the inductor L is relatively lower than that when operated at the CCM.
As shown in the circuit design illustrated in FIG. 5, if the duty cycle of the PWM signal has been designed to satisfy the CCM, the power factor during the CCM can be definitely improved. However, when the PFC circuit 71 is operated at the DCM, the PWM signal continues using the duty cycle for the CCM. When the inductor L is operated outside the duty cycles of the PWM signal and discharges all its stored energy, the energy storage components of the PFC circuit 71, such as diode, boost capacitor and the like, feed back their stored energy to the input terminal of the PFC circuit 71 to cause serious harmonic distortion at the input terminal.
With reference to FIG. 6, to tackle the foregoing shortcoming, an improved PFC circuit 71′ is shown. The PFC circuit 71′ is characterized in a snubber circuit 73 parallelly connected with the active switch 711′ in the PFC circuit 71′. The snubber circuit 73 has a diode 730, a resistor 731 and a capacitor 732. The diode 730 and the resistor 731 are parallelly connected and then are commonly and serially connected with the capacitor 732. The anode side of the diode 730 is connected to the inductor L, and the cathode side is connected to the capacitor 732. If the PFC circuit 71′ is operated at the DCM and within the duty cycle of the PWM signal, part of the stored energy of the energy storage components, such as inductor L, diode D and boost capacitor C of the boost power supply circuit, is absorbed by the snubber circuit 73, thereby effectively reducing the harmonic distortion at the input terminal of the boost power supply circuit.
Whereas, the diode 730, the resistor 731 and the capacitor 732 pertain to passive components, which usually consume energy during the conversion process of the boost power supply circuit, lower the conversion efficiency of the boost power supply circuit, and give rise to low-frequency oscillation to input current.
With reference to FIG. 7, research developers have brought forth a control circuit targeting at determining a currently operating current mode of a switching power supply circuit and varying the duty cycle of the PWM signals corresponding to the present current mode.
Similarly, such control circuit has a rectifier 90 and a PFC circuit 91. An input terminal of the rectifier 90 is connected to an AC mains (Vac). An input terminal of the PFC circuit 91 is connected to an output terminal of the rectifier 90. The PFC circuit 91 has an inductor L, a capacitor C, an active switch 910 and a power factor controller 92.
The rectifier 90 receives a mains power, converts the mains power into a DC power and outputs the DC power.
The input terminal Vin of the PFC circuit 91 is connected to the output terminal of the rectifier 90. The PFC circuit 91 can be operated at the CCM or the DCM. The PFC circuit 91 can be exemplified by a boost power circuit.
Input terminals of the power factor controller 92 are respectively connected to the input terminal and the output terminal of the PFC circuit 91 to acquire an input voltage Vin, sense, an output voltage Vout, sense and an inductor current IL, sense. The power factor controller 92 has an output control terminal connected to the active switch 910.
With reference to FIG. 8, the power factor controller 92 has a computation unit 923 embedded therein and can be operated at a CCM 921 and a DCM 922.
In accordance with the characteristic relation of power factor correction, the computation unit 923 determines at which current mode the power factor controller 92 is operated by comparing two values D1 and D2 expressed as follows.
            D      1        =                                        2            ⁢            L                                              V                                                i                  ⁢                                                                          ⁢                  n                                ,                sense                                      ⁢                          T              S                                      ·                  i          REF                ·                  (                      1            -                                          V                                                      i                    ⁢                                                                                  ⁢                    n                                    ,                  sense                                                            V                                  out                  ,                  sense                                                              )                                D      2        =          1      -                        V                                    i              ⁢                                                          ⁢              n                        ,            sense                                    V                      out            ,            sense                              
If D1>D2, the computation unit 923 determines that the power factor controller 92 is operated at the CCM 921 and switches the power factor controller 92 to be operated at the CCM. A PWM signal having a duty cycle as follows is inputted from the output control terminal of the power factor controller 92.
  D  =            (              1        -                              V                                          i                ⁢                                                                  ⁢                n                            ,              sense                                            V            out                              )        +          [                        L                                    V              out                        ⁢                          T              S                                      ⁢                  (                                    i              REF                        -                          I                              L                ,                sense                                              )                    ]                      where                    L represents the inductor value;            Vin,sense represents the input voltage;            Vout,sense represents the output voltage;            Vout represents the rated output voltage;            Ts represents the cycle of a PWM signal; and            iREF represents a parameter amplified by a proportional integral of an error value between the output voltage Vout,sense and a reference voltage.An analysis of the above duty cycle indicates that the part                        
  1  -            V                        i          ⁢                                          ⁢          n                ,        semse                    V      out      is a theoretical value and the part
      L                  V        out            ⁢              T        S              ⁢      (                  i        REF            -              I                  L          ,          sense                      )  represents a compensation parameter.
If D1<D2, the computation unit 923 determines that the power factor controller 92 is operated at the DCM 922 and switches the power factor controller 92 to be operated at the DCM. A PWM signal having a duty cycle expressed as follows is outputted from the output control terminal of the power factor controller 92.
  D  =                              2          ⁢          L                                      V                                          i                ⁢                                                                  ⁢                n                            ,              sense                                ⁢                      T            S                              ·              i        REF            ·              (                  1          -                                    V                                                i                  ⁢                                                                          ⁢                  n                                ,                sense                                                    V              out                                      )            An analysis of the above duty cycle indicates that the entire portion of
                    2        ⁢        L                              V                                    i              ⁢                                                          ⁢              n                        ,            sense                          ⁢                  T          S                      ·          i      REF        ·          (              1        -                              V                                          i                ⁢                                                                  ⁢                n                            ,              sense                                            V            out                              )      is a theoretical value.
The power factor controller 92 can actively vary the PWM signals based on a currently operating mode of the PFC circuit 91 so as to improve the harmonic distortion arising from the PWM signals mismatching with the operating mode of the PFC circuit 91.
When the PFC circuit 91 is operated at the CCM, the duty cycle contains a theoretical value and a compensation parameter to compensate for deficiency of the theoretical value. However, when the PFC is operated at the DCM, the duty cycle expressed by
                    2        ⁢        L                              V                                    i              ⁢                                                          ⁢              n                        ,            sense                          ⁢                  T          S                      ·          i      REF        ·          (              1        -                              V                                          i                ⁢                                                                  ⁢                n                            ,              sense                                            V            out                              )      and outputted from the power factor controller 92 is merely a theoretical value. If the theoretical value is directly applied to an actual circuit when operated at the DCM, the reducing of the harmonic distortion is not substantial as illustrated by waveforms of the input voltage and input current in FIG. 9.